It is appreciated that tractor-trailers are one of the most efficient and effective cross-country transporting means available today. Tractor drivers log hundreds of thousands of miles over relatively short time periods transporting everything from produce to transformers from coast to coast.
Because of certain regulations and the requirement that a driver sleep and rest, it is common practice for truckers to park their rigs for extended sleep and rest time periods. During these parking periods, it is customary for the trucker to leave the tractor engine "on". This is for various reasons. First, especially at night, the tractor and trailer lights should be maintained "on" for safety purposes. In addition, in cold climates it is necessary for the engine to remain running because in cold climates it can be very difficult to start a tractor engine. In addition, the engine is left running to avoid a power drain of the battery because the tractor operator may be running various accessories in the cab.
The net affect of all of this is that a substantial amount of fuel is required during this "parked" mode. The continuous running of the tractor is a very inefficient means of providing the necessary power for parking lights and other power consuming elements associated with the tractor.
The inefficiency of the above is further compounded in tractor-trailer rigs having a refrigerated trailer. This is because the refrigerated trailer has a refrigeration unit that is powered by an engine that must continuously provide power to drive the refrigeration unit. Thus, in the case of a tractor-trailer with a refrigerated trailer, when the rig is in the "parked" mode, one typically finds the tractor engine and the refrigeration unit's engine both continuously running. That involves a substantial waste of energy and fuel in terms of the load being carried.